1. Field of the Invention
The present invention relates generally to a vacuum system for use in equipment requiring a high, or ultrahigh vacuum, and more particulary, to a double buffer vacuum system for minimizing the vacuum leakage, wherein a high vacuum inner chamber itself and high vacuum components connected thereto are disposed within a rough vacuum outer chamber which is exposed directly to the atmosphere.
2. Description of the Prior Art
With the growth of the machine, electronic and semiconductor industries, vacuum systems, especially high, or ultrahigh vacuum systems, have been widely utilized in the various fields of industry. Representative examples of equipment requiring high, or ultrahigh vacuum systems are vacuum evaporation equipment, sputtering equipment, ion plating equipment, freeze drying equipment, vacuum melting and casting equipment, the heat treatment furnace equipment, etc.
The conventional high vacuum, or ultrahigh vacuum system typically includes a high, or ultrahigh vacuum chamber for producing the high, or ultrahigh vacuum zone, in which a plurality of plate members are welded with each other at their periphery. The vacuum chamber generally has a cooling and heating coil, and a vacuum gauge disposed hermetically in the base plate thereof, and communicates with one end of T-shaped adapter tube. A sleeve-like side flange of T-shaped adapter tube is connected to a rough vacuum pump via a first valve and a rough vacuum line, whereas the other end of T-shaped adapter tube is connected to the rough vacuum pump via a gate valve, a high vacuum pump and a second valve. Thus, the high vacuum can be produced in the high vacuum chamber of the high vacuum system by the rough and high vacuum pumps.
However, in a high vacuum (about 10.sup.-3 to 10.sup.-7 mbar), or ultrahigh vacuum (about 10.sup.-7 mbar or more) system, the high vacuum chamber itself and the high vacuum components connected thereto generally require a high leak-tightness, or sealing ability in order to permit the vacuum leakage not to affect the efficiency of the high vacuum system. Actually, the leak-tightness problem of the high vacuum chamber itself and the high vacuum components connected thereto constitutes the main problem area for high vacuum systems and significantly contributes to the utilization factor in the equipments requiring the high vacuum system.
Heretofore, the conventional high vacuum system has been manufactured to directly expose the high vacuum chamber itself and the high vacuum components connected thereto to the atmosphere. A study revealed that the high pressure difference between the atmospheric pressure and the inner pressure of the high vacuum system is caused by the direct atmospheric exposure of the high vacuum chamber itself and the high vacuum components connected thereto. For example, assuming that the inner pressure of the high vacuum chamber is 10.sup.-7 mbar, the pressure difference between the inside (about 10.sup.-7 mbar) and outside (about 1013 mbar) of the high vacuum system is almost one atmosphere. This high pressure difference may causes the following problems as follows:
(1) It is possible that large amounts of air gas molecules may penetrate into the high vacuum chamber through the microscopic defects in the welded portions of wall members if the welding is poor.
(2) The high vacuum system may leak large amounts of gas molecules in locations where high vacuum components, including door, the are connected with the high vacuum chamber by means of a sealing means such as O-rings.
In theory, the leakage quantity per second Q occurs due to pressure diff between the inside and outside of the high vacuum system generally is given by following formula; EQU Q=(P.sub.1 -P.sub.2).times.C
where P.sub.1 is the outside pressure of the high vacuum system, P.sub.2 is the inside pressure of the high vacuum system, and C is the conductance of leak which is determined by the shape and size of the leak cause part of the high vacuum system. From this formula, it becomes more clear that the pressure difference between the inside and outside of the high vacuum system constitutes an important factor capable of causing the vacuum leakage.
In addition to the leak-tightness problem, an important factor effecting the efficiency or utilization factor of the high vacuum system is `out-gassing` of the inner surface of the high vacuum chamber which can occur during operation of the high vacuum system. This may be reduced by inner surface treatment condition and the type of materials used in the wall members of the high vacuum chamber. Therefore, it is necessity to perform an inner surface treatment process, such as degreasing, polishing, coating, or ball blasting of the inner surface of the high vacuum chamber, after a plurality of wall members of the high vacuum chamber are welded with each other at their periphery. This inner surface treatment process has been proven to be difficult because it is performed with the spacial restriction of the high vacuum chamber after welding.
Consequently, a need exists for an effective system to minimize the vacuum leakage and to enable the inner surface treatment process, cleaning, or repairs to be easily performed, so as to improve the reliability and the efficiency thereof.